1. Field of the Invention
The present invention relates to a control system for an automatic transmission of a motor vehicle, and more specifically to a line pressure control for a hydraulic control mechanism of the transmission.
2. Description of the Prior Art
Generally, an automatic transmission of a motor vehicle is provided with a torque converter, and a transmitting mechanism employing a planetary gear mechanism. The transmission is also provided with a plurality of frictional elements, such a clutches and brakes, for establishing a desirable shift gear stage among the plurality of shift gear stages provided in the transmission. For this purpose, a hydraulic control circuit is provided with a plurality of solenoid valves for switching oil passages in the circuit so that the frictional elements are engaged and disengaged to perform a desirable shift operation.
Japanese Patent publication No. 54-2349, issued to Ford Motor corporation and published for opposition on Feb. 6, 1979, discloses a basic electrical hydraulic control system for an automatic transmission.
Switching operation of the oil passages of the hydraulic circuit must be timely done; otherwise, there might be produced a torque shock resulting form unusual change in engine and/or turbine speed.
In view of the above, there has been proposed controlling engaging force of the frictional elements to obviate the torque shock during a shift operation of the transmission. For instance, Japanese Patent Public Disclosure No. 56-10851, laid open to the public in 1981, discloses a hydraulic control in which the hydraulic pressure introduced to the frictional element is controlled to gradually change the engaging force of the frictional elements and controlled to reduce the difference between a target time period for shift operation and an actual time period thereof.
Japanese Patent Publication Nos. 49-30051, published for opposition on Aug. 9, 1974, and 61-48021, corresponding to U.S. Pat. No. 4,502,354 and published for opposition on Oct. 22, 1986, disclose similar control system for obviating the torque shock due to shift operations.
However, the conventional hydraulic control mechanisms as such disclosed in these Japanese Patent applications are disadvantageous in that the hydraulic pressure control based on the shift operation time period is not accurate, and thus not so effective in controlling the speed change of the engine and/or the turbine.